In the present description, the transmission apparatus, the relay apparatus, and the receiving apparatus may be referred to as a transmission node, a relay node, and a receiving node respectively.
There has been considered a communication system that utilizes a plurality of relay nodes present between a transmission node and a receiving node to thereby improve the quality of communication, in a communication system, particularly, in a wireless communication system. FIG. 1 is a diagram for describing a configuration of a communication system that utilizes a plurality of relay nodes. The communication system shown in FIG. 1 shows a transmission node 100, a first relay node 200-1, a second relay node 200-2, and a receiving node 300. The first relay node 200-1 and the second relay node 200-2 are collectively referred to as relay nodes 200 when they are not differentiated. The number of relay nodes 200 in the communication system may be an arbitrary number not less than 1. The connection between the transmission node 100 and the relay nodes 200, and the connection between the relay nodes 200 and the receiving node 300 may be either one of wired connection and wireless connection.
There has been a communication system in which a likelihood detected in a relay node in communication between a transmission node and a relay node is quantized and forwarded to a receiving node. Furthermore, there has been considered a relay system in which quantization parameters are adaptively changed according to the probability distribution of a detected likelihood, in order to reduce likelihood quantization bits (for example, refer to Non-Patent Document 2).
There has been considered a configuration in a wireless communication system, particularly, in a mobile communication system, such that a plurality of antenna units equipped with some base station functions are physically stretched out and these antenna units are used as distributed stations. An antenna unit refers to a unit that is equipped with a transmission interface, a wireless transmitter/receiver, and an antenna. A base station with distributed stations stretched out therefrom functions as a central station to control the plurality of distributed stations.
FIG. 2 is a diagram for describing a configuration of a communication system that uses likelihood adaptive quantization in the case where the relay nodes and the receiving node are connected by means of wired connection. The transmission node 100, the relay nodes 200, and the receiving node 300 in this case can be perceived respectively as a terminal, distributed stations, and a central station in the mobile communication system described above where the base station functions are divided into the central station and the distributed stations.
First, a signal transmitted from the transmission node 100 is received at a wireless receiving unit 21 of the relay node 200. The signal gets noise added thereto. A soft demodulation unit 22 of each relay node 200 performs soft demodulation on the received signal, to thereby detect the likelihood of the received signal. Next, a quantization parameter calculation unit 23 calculates a quantization parameter (what type of quantization width to be employed) by observing the probability distribution of the detected likelihood. A likelihood adaptive quantization unit 24 performs likelihood adaptive quantization according to this quantization parameter. For example, a quantization parameter may be a quantization level value, which is a value after likelihood has been quantized.
The quantized likelihood is transmitted as a likelihood quantized value, which is expressed as a bit of ‘0’ or ‘1’, from the wired transmission unit 25 of each relay node 200 to the receiving node 300. The receiving node 300 is yet to know what type of quantization parameter is used in quantization of the transmitted likelihood. Consequently, the relay node 200 needs to notify the receiving node 300 of the quantization parameter as well as the likelihood quantization bit value. In FIG. 2, reference symbol R1 denotes a signal that includes a likelihood quantization bit value and a quantization parameter (the same applies to the example in FIG. 3). Finally, an inverse quantization unit 32 of the receiving node 300 performs an inverse quantization process to determine the received bit, based on the notified quantization parameter, and the signal is forwarded to an upper layer function unit 33.
FIG. 3 is a diagram for describing a configuration of a communication system that uses likelihood adaptive quantization in the case where the connection between the relay nodes 200 and the receiving node 300 is wireless connection. In this case, the process up to performing likelihood adaptive quantization in the relay node 200 is similar to that in the configuration of FIG. 2. A likelihood quantization bit value is modulated in a wireless transmission unit 26, and is transmitted as a wireless signal to the receiving node 300. In the receiving node 300, each wireless receiving unit 34 receives a signal from each receiving node 200. A demodulation unit 35 demodulates the received signal, and the inverse quantization unit 32 performs the inverse quantization process on the demodulated signal. At this time, the demodulation process may be either one of hard determination and soft determination.
The communication systems shown in FIG. 2 and FIG. 3 enable improved communication quality, compared to that in performing direct communication between the transmission node and the receiving node.